The present invention relates to an adjusting mechanism for pivotal guide blades of turbo-propulsion units with an adjusting ring disposed outside of the turbine housing which is rotatably supported by way of bearings at a coaxial support ring and on which pivot levers of the guide blades bendable perpendicularly to the pivot plane on the blade side are movably arranged in four degrees of freedom.
Such an adjusting mechanism is disclosed in the U.S. Pat. No. 2,933,234. The adjusting ring is thereby supported by way of slide members at concentrically inwardly disposed support ring segments which, in turn, are secured at the housing of the turbo-propulsion unit. The bendable pivot levers of the guide blades are supported in the adjusting ring by way of ball joints. This arrangement has as a consequence that the propulsion unit heat is transmitted unobstructedly onto the adjusting mechanism whereby large temperature differences result between the start and operating phase of the propulsion unit with correspondingly high thermal expansions of the adjusting mechanism. In order to be able to absorb the same, sufficient elasticities or clearances must be provided in the adjusting mechanism. This, in turn, leads to inaccuracies of the guide blade adjustments by reason of the large actuating forces to be transmitted which are then connected with efficiency losses. It may even lead to mechanical damages, for example, as a result of vibrations.
It is the object of the present invention to avoid these disadvantages and to assure an exact adjustment of the guide blades independently of the temperature fluctuations caused by the differing operating conditions.
The underlying problems are solved according to the present invention, in that the support ring is connected heat-insulatingly with the turbine housing by way of several connecting lugs distributed over the circumference.
It is achieved by this arrangement that the heat flow from the turbine housing to the support- and adjusting-ring remains relatively small independently of the operating condition of the propulsion unit and the same retains an approximately constant temperature. As the turbine housing is generally surrounded by a cooling air stream, the heat conduction by way of the connecting lugs is further restricted.
Small thermal expansions in the axial direction of the propulsion unit can be absorbed by the lugs without errors for the blade adjustment whereas the coaxiality of the support ring and propulsion unit remains assured.
Preferably, the connecting lugs are flat sections of an annular band. A surface-/cross-section ratio can be achieved thereby favorable for reduced heat conduction. Additionally, the connecting lugs have a certain elasticity in the radial direction, as a result of which the differing thermal expansions of the turbine housing which becomes hot during operation and of the adjusting- and support-ring which remain cool can be compensated for.
In a particular embodiment of this invention, the connecting lugs are connected with a fastening ring secured at the turbine housing. As a result thereof, a simple and accurate assembly of the adjusting mechanism can be achieved.
In a further embodiment of this invention, the support ring, the fastening ring and the connecting lugs are combined into a bearing ring which is constructed as an integral component which leads to a simplification of the manufacture.
The bearing ring, according to another embodiment of the present invention, may consist of two or several ring segments connected with each other whereby the fastening of the adjusting mechanism is facilitated.
In an alternative embodiment of the invention, the support ring is connected with the turbine housing by way of several slide block guidances distributed over the circumference. As a result thereof, the support ring is movable in the axial direction of the propulsion unit and can thus absorb advantageously differing thermal expansions of the support ring and of the housing. Additionally, the heat flow from the housing into the ring is reduced.